Protective coating for magnesium

ABSTRACT

A METHOD OF PRODUCING HARD PROTECTIVE BLACK COATINGS ON MAGNESIUM AND MAGNESIUM BASE ALLOYS. THE METHOD INCLUDES MAKING THE MAGNESIUM ARTICLES THE ELECTRODES AND ELECTROLYTICALLY TREATING THEM UNDER PROGRESSIVELY INCREASING POTENTIAL IN AN AQUEOUS SOLUTION COMPRISING A CHROMATES, A VANADATE, A PHOSPHATE AND A FLUORIDE. THE PH OF THE SOLUTION IS MAINTAINED WITHIN A RANGE OF ABOUT 5.5 TO ABOUT 9 WITH AN ALKALINITY AGENT YIELDING NON-METALLIC CATIONS ONLY.

United States Patent PROTECTIVE COATING FOR MAGNESIUM William-McNeil], Southampton, Pa., assignor to Duromag Coatings Inc., Philadelphia, Pa.

No Drawing. Filed Sept. 21, 1972, Ser. No. 291,108 Int. Cl. C23b 9/06 US; Cl. 204-56 M 10 Claims ABSTRACT OF THE DISCLOSURE A method of producing hard protective black coatings on magnesium and magnesium base alloys. The method includes making the magnesium articles the electrodes and electrolytically treating them under progressively increasing potential in an aqueous solution comprising a chromate, a vanadate, a phosphate and a fluoride. The pH of the solution is maintained within a range of about 5.5 to about 9 with an alkalinity agent yielding non-metallic cations only.

, This invention relates to a method of coating magnesium' and magnesium base articles, and more particularly, to a method of providing a black protective coating on the articles by anodic electrolytic treatment.

Magnesium and magnesium-base alloys are highly reactive metals, and are easily corroded. For this reason, they must be protected. Various methods have been developed in the prior art to place protective coatings on the magnesium.

One such method of protecting magnesium is disclosed and claimed in my prior US. Pat. No. 2,778,789. This patent teaches that protective coatings can be applied to magnesium and its alloys by anodic electrolytic treatment in aqueous ammoniacal solutions containing chromate, phosphate and fluoride ions, and from which alkali metal ions are excluded. Coatings are obtained from these solutions with direct or alternating current and with the latter, applied voltages of the order of 300 volts are typical for fully formed coatings. The coating process is characerized by vigorous sparking on the surfaces of the magnesium articles being coated and this sparking creates a local thermal environment in which the electrolyte constituents and the magnesium unite chemically to form ceramic coatings which are tightly bonded to the magnesium surface.

In the process of my US. Pat. No. 2,778,789 the coatings obtained are exceptionally smooth, having surface roughness values of the order of 40 to 100 microinches RMS. These same coatings provide a very marked degree of protection for the magnesium against salt spray corrosion.

The color of coatings described in my prior patent may range from light green to a very dark green, which is nearly black, depending on the concentration of chomate ion in the electrolyte solution. Solutions having chromate concentrations up to 0.7 mole per liter are claimed, and these concentrations of chromate ions are known to produce nearly black, lusterless coatings on magnesium and its alloys.

Another prior art method of electrolytically coating magnesium is disclosed in US. Pat. No. 2,880,148. In this method, the magnesium or its alloys are coated electrolytically in a bath containing fluorides, phosphates, alkali metal hydroxides and a compound selected from the class consisting of chromates, selenates, stannates, tungstates, vanadates, molybdates, silicates and borates. I

The coating method of my present invention possesses a number of advantages over the prior art coating methods referred to above. The method of my present invention produces a pure black coating. The low reflectance of the black makes it excellent for photo-optical applications, such as cameras, projectors and binoculars. Addi- 3,791,942 Patented Feb. 12,, 1974 tionally, the black coating is far more protective than the coatings produced by the prior art processes, and therefore it is not necessary to further treat or coat the magnesium articles that have been coated by the method of my present invention.

The color of the prior art coatings from electrolytes containing 0.7 mole per liter chromate are dark green, rather than a true black. At this, and higher chromate concentrations, there is an increasing tendency for cratered nodules to form in the coating. This not only detracts from the appearance and smoothness of the coatings, but also creates a source of powdery dust which is objectionable in optical equipment.

Insofar as the process described in US. Pat. No. 2,880,- 148 is concerned, the coating formation time in the present invention is about one-fifth that required in US Pat. No. 2,880,148. For example, in the present invention, 200 ampere minutes per square foot are required for a high degree of protection, whereas in the process in US. Pat. No. 2,880,148, 1000 ampere minutes per square foot are required to achieve the degree of protection claimed. In addition, the coatings described in this patent require postanodic dips in chromate-bifluoride solutions and humid oven aging to achieve the degree of protection claimed. The present invention dispenses with these steps.

It has been found that in salt spray corrosion tests, the coating of this invention performed approximately three to five times as well as the coating prepared according to the method described in US. Pat. No. 2,880,148.

It is therefore an object of this invention to provide a lusterless, smooth, black protective coating on magnesium and magnesium alloys.

It is another object of this invention to provide a method of coating magnesium and magnesium alloys, which coating is free of nodules and other irregularities.

It is a further object of this invention to provide protective coatings for magnesium and magnesium alloys which exhibit a high degree of hardness and abrasion resistance.

These and other objects of this invention are accomplished by providing a method of producing protective black coatings on magnesium and magnesium base alloy articles which comprises making the articles the electrodes and electrolytically treating them in an aqueous solution comprising a chromate, a vanadate, a phosphate, and a fluoride, said solution yielding only non-metallic cations, and said solution having a pH that is maintained in the range of about 5 .5 to 9.0 with an alkalinity agent yielding non-metallic cations only. The process is carried out in the same manner as that described in my aforementioned US. Pat. No. 2,778,789, the disclosure of which is incorporated by reference herein.

The articles to be coated are made the electrodes and alternating current is applied therethrough to the bath. Direct current may also be employed, but is not preferred because somewhat greater difliculty is encountered in obtaining coatings therewith. The articles are first suitably cleaned, and thereafter placed in the electrolytic bath.

Preferably, articles of approximately equal surface area electrodes), a temperature of at least 50 F., and a treatment time of from two to fifteen minutes. As a rule it is preferable to keep the product of the time and current density constant, so in case one is raised or lowered it is.

best to inversely lower or raise the other. A current density/treatment time product of 200 ampere minutes per square foot provides excellent coatings.

The current density can be varied widely, from as low as 5 amps per square foot to as high as 200 amps per square foot, with success, I rent densities could be used so as to speed up the process.

.Ihssleetrvl p s lu on ont s s b on c stituents which when incorporated into the protective coatings by the anodic process described above provide therein a multiplicity of multivalent metal ions which substantially or interchangeably occupy the same type of lattice sites in the glassy and crystalline phases that comprise the ceramic protective anodic coatings. The multivalent metal ions which are suitable for the purposes of this invention are typically colored species, that is, species which absorb electromagnetic radiation in the visible portion of the spectrum. In order to achieve a black, lusterless coating it is important to absorb incident radia tion throughout the visible region and have no part of the visible spectrum strongly reflected by the coating. It is thus necessary that the multivalent ionic absorbers be distributed more or less uniformly throughout the ceramic coating constituents, particularly in the regions nearest the outer surface of the coating.

The purpose of utilizing a multiplicity of multivalent metal ions in such coatings is to obtain optical absorption more fully over all of the visible spectrum, and to obtain enhanced optical absorption due to interaction broadening of electronic energy levels in the absorbing metal ions. Thus, if predominantly red-orange absorbing, and predominantly blue-green absorbing ions are combined in the same ceramic crystal lattices and glassy phases in the proper proportions, the net effect is a general absorption of visible radiation which leads to a black appearance in the ceramic coating. For example, in the prior art ammoniacal chromate, phosphate, fluoride electrolytic treatment for magnesium the resultant ceramic anodic coatings are green with the depth of color increasing with chromate concentration. At the chromate concentrations of the order of 0.7 mole per liter the chromium ions in the resultant coatings quite probably exist in several valence states.

If the chromate ion is deleted and vanadate ion is added to the electrolyte in the form of soluble ammonium vanadate, the resulting anodic coatings are yellowbrown with the depth of color increasing with vanadate concentration. If chromate and vanadate ions are both present in the electrolyte, coatings can be obtained which range in color from green to brown, but with a wide range of solution compositions in which the coatings are black. In fact, the presence of a relatively small amount of vanadate, in a solution containing chromate ions in concentration only suflicient to yield green coatings, caus es considerable darkening of the otherwise green coating, rendering it dark brown or black. This elfect is observed when the concentration of the vanadate is only sufficient to produce light tan coatings and the concentration of chromate is only sufficient to produce olive green coatings.

A specific embodiment of an anodic treatment solution that can be used in carrying out this invention is shown in the following example:

' The foregoing solution is used to obtain smooth black ceramic coatings on magnesium and its alloys.

Equivalent materials can be substituted for the materials shown in Example 1, while at the same time obtaining the same results. Thus, the anodic treatment solution may also be formulated from hydrofluoric acid (HF), ammonium fluoride (NHQF);- ammonium di-hydrogen and undoubtedly higher curi .4 i Z. phosphate (NH H PO phosphoric, acid (H POr), ammonium chromate ([NH CrO ammonium dichromate ([NHd cr O ammonium metavanadateand ammonium hydroxide (NH OI-I) in quantities which yield the equivalent fluoride, phosphate, chromate and vanadate concentrations and pH of the solution shown in Example 1 or solutions of compositions within the limits shown below.

In order to obtain such coatings the magnesium articles to be coated are made electrodes in the above solution and alternating electric current is passed through them and through the electrolyte. As pointed out above, articles of approximately equal surface area are connected to each bath terminal in the AC. operation. Electric current densities (calculated as the RMS electric current passing through the bath divided by the total surface area of magnesium being coated in the bath) may range from 5 amperes per square foot to over 200 amperes per square foot, with black coatings being obtained over the entire range. Solution temperatures may range from 50 F. to the boiling point of the solution which is approximately equal to that of water, and excellent black coatings may be obtained over the entire temperature range. Treatment time is of the order of two to ten minutes, depending on current density. Current density and treatment time products in the range from 100 ampere minutes per square foot to 300 ampere minutes per square foot generally produce the most satisfactory coatings. The more extensive treatment times produce generally thicker coatings, which are also somewhat rougher than those produced with less treatment time. The slight roughening of the coating is not at all prominent.

The solution composition may range widely also. If fluoride and phosphate concentrations are set at the values given above for the preferred composition, and chromate and vanadate concentrations are allowed to vary, coatings having colors ranging from green to brown and including black may be obtained With chromate concentrations in the range of 1.0 gram per liter to grams per liter of CrO or more. Simultaneously, vanadate concentrations may range from 2.0 grams per liter to 50 grams per liter of V 0 In the higher concentration solutions the ammonium added must be sufficient to leave the solutions approximately neutral in pH, or slightly alkaline. In a preferred embodiment the chromate concentration is equivalent to 35 grams per liter of CrO and the vanadate concentration is equivalent to 15 grams per liter of V 0 The fluoride concentration can range from 35 ml./l. to 45 ml./l. of hydrofluoric acid (48% assay), or an equivalent amount of salt. The phosphate concentration can range from 40 ml./l. to 60 ml./l. of phosphoric acid assay), or an equivalent amount of salt.

The ammoniacal chromate, vanadate, phosphate, fluoride electrolytic treatment for magnesium offers several decided advantages over the prior art. The black color is attained with far less chromate concentration than is required in the prior art processes. As a consequence, the coatings are a more uniform black color and are free of cratered nodules which characterize coatings obtained in solutions having chromate concentrations equivalent to more than 60 grams per liter of CrO Furthermore, in the process of this invention, uniform, smooth, black, protective coatings, which are virtually identical in appearance, are obtained over a very wide range of processing conditions. Excellent black coatings may be obtained with current densities ranging from 5 amperes per square foot to over 200 amperes per square foot. Coatings obtained with electrolyte temperatures ranging from 50 F. to the boiling point of the electrolyte are uniformly black in color. Similarly, processing time may be extended from ampere minutes per square foot to several times that value, and the resultant coatings may range from about 0.4 mil to about 1.1 mils.

thick, while still retaining a uniform smooth black appearance, and .areindistinguishable from one another.

In contrast, the. coatings obtained inthepriorart processes, utilizing only chromate,,phosphate, and fluoride in ammoniacal solution are very sensitive to chromate concentration and to process-conditions; For example',.with chromate concentrations equivalent to less than about 60 grams per liter of CrO the resultant anodic coatings are a ,darkolive green. As chromate concentrations are increased to 70 grams per liter CrO or higher, which is necessary to obtain black-or nearly black coatings, the tendency to form non-uniform coatings becomes pronounced and can be corrected only by operating at current densities of the order of 75 amps per square foot.

Similarly, in the prior art processes, while coatings can be obtained with electrolyte temperatures below 70 F., they are typically thin, less than 0.2 mil in thickness, and are densely interspersed with nodules.

Even more striking is the difference in the effect of extent of anodic treatment. In the prior art processes, extending the treatment to formation voltages beyond that required for optimum dark green or black coatings results in a lighter green coating. Thus, on complex parts, where current densities may vary considerably from convex to concave areas it is still possible with the process of this invention to maintain a uniform black coating, whereas in the prior art processes, some areas on complex pieces acquired a green color if treatment is prolonged sufficiently to attain dark color on deep concave surfaces. This uniformity of color is a substantial improvement over the prior art.

In addition to providing protective coatings on magnesium, the ammoniacal chromate, phosphate, fluoride, vanadate solution can be used for the electrolytic formation of protective coatings on zinc and zinc alloys. In order to coat zinc articles, a solution having the composition given in Example 1 may be used. The zinc articles to be coated are immersed in the solution, and connected to the terminals of the electrolytic treatment tank. Alternating electric current is passed through the electrodes and through the solution at current densities of the order of 100 to 200 amperes per square foot, for periods of five or ten minutes, or more, with the resulting formation of a dark gray-green ceramic protective coating on the zinc articles.

In View of the foregoing detailed description of my present invention, a more complete comparison can be made with the method and solutions disclosed in U.S. Pat. No. 2,880,148. In the prior patent, all of the solutions used are composed of alkali metal salts and alkali metal hydroxides in concentrations such that a substantial quantity of free alkali is always present. Therefore, the prior art solutions are always strongly alkaline and will have pH values in excess of 12.0. In contrast, the present invention uses strictly alkalinity agents which yield non-metallic cations only. The pH is not in excess of 9.0 for the preferred embodiments of the solution of my invention. In fact, introduction of even trace amounts of alkali metals in the present invention causes roughness in the coating and formation of undesirable nodules.

The chemical compositions of the prior art solutions disclosed in U.S. Pat. No. 2,880,148 and my present invention are mutually exclusive. One cannot make the present invention solutions by substitution of ammonium hydroxide in place of sodium or potassium hydroxide in the prior art because many of the constituents of the prior art solution would precipitate, that is, aluminate, tungstate, silicate, stannate and molybdate precipitate at pH ranges which occur in ammoniacal solutions. Also, the addition of ammonium hydroxide to the prior art solution would result in decomposition of the ammonium salts and evolution of ammonia gas.

The present invention teaches a multiplicity of multivalent metal ions in the solutions and in the coating. The prior art teaches that any one of a group of metal containing ions may be used. The prior art makes no refercache the optical absorption spectraof the ceramic coating constituents resulting from the anodic deposition process. None of the compositions given in the prior art yield'black coatings. I

In all cases in the prior art, sodium or potassium salts aresubstituted for one another, or selenate, stannate, tungstate, vanadate or molybdate are substituted for chromate. It' is important to note that in the prior art,'vanadate is substituted for chromate, and not used in conjuncion therewith.

The coating formation time in the present invention is about one-fifth that required in the prior art, as exemplified by U.S. Pat. No. 2,880,148, for a comparable degree of protection. For example, in the present invention 200 ampere minutes per square foot are required for a high degree of protection. In the prior art approximately 1000 ampere minutes per square foot are required for equivalent protection. In addition, the prior art coatings require post-anodic treatment dips in chromate-bifiuoride solutions and humid oven aging to achieve the degree of protection claimed. The present invention dispenses with these steps.

The exact nature of the coating on the magnesium article is not known. However, it is believed to be an adherent coating comprising a mixed crystalline and glassy phase ceramic composition comprising the insoluble oxides and fluorides of magnesium, phosphorous, vanadium and chromium, and their derivative compounds.

Without further elaboration, the foregoing will so fully illustrate my invention that others may, by applying current or future knowledge, readily adapt the same for use under various conditions of service.

What is claimed as the invention is:

1. A method of producing protective coatings on magnesium and magnesium base alloy articles comprising making the articles the electrodes and electrolytically treating them in an ammoniacal aqueous solution free from alkaline metal ions wherein the pH is maintained in the range of 5.5 to 9.0 and comprising chromate, vanadate, phosphate and fluoride,

the current density treatment time products being in the range of ampere minutes per square foot to 300 ampere minutes per square foot,

the treatment time being of the order of two to ten minutes, and

the temperature of the solution being in the range from 50 F. to the boiling point of the solution.

2. The method of claim 1 wherein the fluoride is present as NH HF and the phosphate is present as Qz -i 3. The method of claim 1 wherein the chromate concentration is in the range of 1.0 gram per liter to 75 grams per liter of CrO and the vanadate concentration is present in the range of 2.0 grams per liter to 50 grams per liter of V 0 4. The method of claim 1 wherein the chromate concentration is equivalent to 35 grams per liter of CrO and the vanadate concentration is equivalent to 15 grams per liter Of V205.

5. The method of claim 2 wherein the pH is maintained by adding NH OH to the solution.

6. The method of claim 2 wherein the fluoride, phosphate, chromate and vanadate are in the following proportions per liter of solution:

Amount per liter of solution 8. The method of claim vl lv'vliere'in range from 5 amperes per square foottoapproxima tely 5 200 amperes per square foot. U

9. The method of claim 1 wherein the chromate concentration is in the range of L0 gram per liter to 75 grams per liter of CrO and the vanadate concentration is present in the range of 2.0 grams per liter to 50 grams per liter of V205. ii I I 10. A magnesium or magnesium bas e alloy article having a hard, protective, black coating thereon, said coating being applied by the method of claim 1.

8 References Cited 2,880,148 3/1959 'Evangelides '204 s6 M OTHER REFERENCES A.P.C. Application of Frasch, Ser. No. 297,191, Published May 18, 1943. I

A.P.C. Application of Frasch, Ser. No. 257,762, Published June 22, 1943. i i

THOMAS M. TUFARIELLO, Primary Examiner 

